When burning coal, the nitrogen content in the fuel organic mass and the nitrogen from the combustion air are oxidized to form nitrogen oxides (NOx) in the exhaust, or flue gas. There are different methods for reducing the nitrogen oxides formed in a coal combustion process. They are achieved using air staging, burner staging and fuel reburning, as well as many others. The general concept is to create a substoichiometric zone in the furnace, where different radicals are formed, including CN, OH, among others. These radicals at certain ranges of flue gas temperature and residence time promote reactions which will reduce the NOx formed in the flame to N2.
Normally, in the pulverizing systems for the combustion of subbituminous and bituminous coals, different types of ring-roll or ball-race mills are used. In general, these are direct-fired pulverized coal systems where the drying and transportation agent is primary hot air, heated by the air preheater and redirected back to the pulverizer.
Possible fires and explosions in pulverizers and in the duct system after pulverizers are controlled by maintaining the temperature within certain limits depending on the type of coal. To prevent explosions when fire occurs in the pulverized coal duct system, inert agents such as carbon dioxide (CO2), nitrogen (N2) or steam are used which are injected into the pulverizer/coal duct after the pulverizer.
FIG. 1 depicts a block diagram of the architecture of a typical bituminous or sub-bituminous coal burning power plant. Specifically, FIG. 1 depicts a boiler system 100 of the prior art including a boiler 102, an air preheater 106, an electro-static precipitator 104 and a pulverizer 110. In operation, flue gas created by the coal flame 120 in the boiler 102 travels through a flue gas duct 116 to the air preheater 106, where the flue gas is cooled and further sent through the flue gas duct 116 to the electro-static precipitator 104. The electro-static precipitator 104 is configured to remove any particles or pollutants from the flue gas such as fly ash, and then discharges the flue gas from the boiler system 100.
Still referring to FIG. 1, outside air is brought into the boiler system 100 through the cool air duct 114. Depending upon the temperature requirements of the boiler system 100, a portion of this air will be heated with the air preheater 106, while the remainder of the air travels through the cool air duct 114, and is injected into the pulverizer 110. The outside air that is heated by the air preheater 106 travels through the hot air duct 112 and is split, a portion traveling to the boiler 102 for combustion, while the remainder is injected into the pulverizer 110 to adjust the temperature inside the pulverizer 110 as required by the boiler system 100. The fuel port 108 delivers coal to the pulverizer 110 and the pulverized fuel duct 118 delivers the pulverized coal and outside air mixture to the boiler 102 where it is burned in the coal flame 120.
The boiler system 100 depicted in FIG. 1 embodies the problems described above as the combustion of the mixture of outside air and coal creates a relatively large amount of nitrous oxide that is then discharged with the flue gas. Furthermore, the outside air in this mixture contains a significant amount of oxygen, which can make the fuel/air mixture coming out of the pulverizer flammable and explosive.